This invention is directed to dry film photoresist constructions and in particular is directed to dry film photoresist constructions which suitably reside in roll form. This invention eliminates the currently used polyethylene protective sheet which is a cause of many photoresist defects and yet can still be applied in a traditional manner. The dry film photoresist of this invention is useful as will be apparent to those skilled in the art for making printed circuit boards such as shown in U.S. Pat. Nos. 5,213,945 and 4,992,354.
A dry film resist is not truly a dry film but rather a very highly viscous liquid which is sandwiched between a carrier sheet and a protective sheet and is subject to flow over the course of time particularly if there are internal stresses from this construction that force the resist to flow. When internal stresses in a typical dry film photoresist cause the resist to flow, the results are thin spots, thin lines, or other areas where film thickness is less than desired as well as areas as the end of the roll when the resist flows out and fuses with other areas of flow. This generates resist chips when unrolled causing other types of defects.
One of these stresses results from the polyethylene (PE) protective sheet currently used in most dry film constructions. Some of these stresses result from gel particles that are typically induced in the PE sheet during the "blown film" process and reside as a protrusion in the surface of the sheet. In the dry film photoresist structure these gel protrusions cause a stress within the roll and cause the highly viscous liquid to flow away from that area to result in a thin spot or ultimately, in a void. This defect transfers itself to the copper innerlayer substrate as a thin coating area on an open area and during such processes, e.g. expose, develop and etch processes or expose, develop and plating processes, the thin spots function as weaknesses in the resist. This results in etchant attack and causes at the very least a thinness of copper lines (neckdown) or a missing portion of a continuous line (mouse bite, effecting impedance control) or at the very worst a line breakage (open). In plating processes these defects result in shorts. Each of these types of defects in the resist reduce and effect the resultant yield of the circuit board producer.
The other area where gel particles in the PE create a limitation in the dry film process is in how thin the coating can be in order to produce finer and finer lines. The thinner the dry film coating the more pronounced will be the effect of the gel. If a coating is already thin, the gel does not have to put on it too much stress to cause the resist to flow. In cases where the dry film is very thin, about 1.0 mil and below, the resist can flow enough to cause a complete loss of coating resulting in a void which will create an open, or cut line on plating or a short, in the final circuit board obtained after processing.
Other types of cover protective films are less desirable due to cost or their inability to function in a rolled up capacity. Complete removal of the PE and allowing the polyethylene terephalate (PET) layer covered resist to roll up against itself is problematic in that the resist has no memory and over time will adhere to the "backside" of the PET as well as the "front" side. Also waste treatment of PE can be an added cost to circuit board production. For further background information, see for example U.S. Pat. Nos. 5,213,945 and 4,992,354 cited above for a disclosure of dry film photoresist products and photoresists therefore. The entire contents of these patents are incorporated herein by reference hereto.